SUMMARY

The present study reports the first use of a choice visual–vocal
response time cognitive task, during 72 or 120 h of continuous auditory
vigilance. Two adult bottlenose dolphins (Tursiops truncatus), NAY
(male) and SAY (female), maintained a very high detection rate
(91.1–98.7%) of random 1.5 s goal tones infrequently substituted in a
background of frequent 0.5 s equal-amplitude tones over continuous 72 or 120 h
sessions. In addition, a choice visual–vocal response time task (CVVRT)
tested cognitive performance during night time sessions, when the dolphins
would have ordinarily been resting or asleep as we had observed in previous
studies. NAY and SAY detected a single-bar, posterior, vertical, green (S1g)
or 3-bar, anterior, horizontal, red (S2r) LED light stimulus presented
randomly to each eye. They responded with a different vocalization (whistle or
pulse burst) to each stimulus (S1g or S2r) presented randomly to left and
right eyes. The animals maintained high levels of goal tone detection without
signs of sleep deprivation as indicated by behavior, blood indices or marked
sleep rebound during 24 h of continuous post-experiment observation. Acoustic
goal tone response time (AGTRT) overall did not change during the 72 h
(F=0.528, P=0.655) or 120 h (F=0.384,
P=0.816) sessions. Nor did CVVRT slow or degrade over the 72 h
(F=4.188, P=0.104) or 120 h (F=2.298,
P=0.119) AGTRT sessions.

INTRODUCTION

A previous study (Ridgway et al.,
2006), reported for the first time that bottlenose dolphins
(Tursiops truncatus) could maintain auditory vigilance for 120
continuous hours. These animals showed no evidence of sleep rebound or health
effects of sleep deprivation as indicated by behavior, blood indices or marked
sleep rebound during 24 h of continuous post-experiment observation. The two
dolphins maintained a very high detection rate (87–99%). They detected
randomly presented, infrequent 1.5 s goal tones in a background of frequent
0.5 s equal-amplitude tones over five continuous 120 h sessions. Auditory goal
tone response time (AGTRT) did not slow between day one and day five whereas
AGTRT was slower during night time (21:00–04:00 h) compared with daytime
(09:00–16:00 h). However, the dolphins were able to maintain levels of
vigilance as indicated by reliable goal tone detection even during night time
periods of slowed AGTRT when the animals appeared to be resting or sleeping
(Ridgway et al., 2006).

The dolphins had to detect the infrequent, randomly presented 1.5 s goal
tone every 4–24 min against a background of equal amplitude, equal
frequency 0.5 s tone presented every 30 s. The animal responded to the goal
tone by swimming to and pressing a response paddle at the side of its
enclosure. This AGTRT task was sufficient to show that the dolphins could
maintain auditory vigilance for a continuous 120 h without signs of sleep
deprivation or health effects (Ridgway et
al., 2006). However, the task did not test the dolphins' cognitive
ability with the progression of continuous vigilance.

Human cognitive ability, as measured by choice response time, degrades more
rapidly than physical performance during periods of continuous vigilance
(Dinges et al., 1997;
Liberman, 2006;
Liberman et al., 2006). To
test whether this is also true in dolphins, we introduced a dolphin night time
cognitive task superimposed on the AGTRT. This would be a repetition of the
same auditory vigilance task (Ridgway et
al., 2006) with the addition of a night time choice
visual–vocal response time (CVVRT) task. We hypothesized that during
72–120 h of continuous auditory vigilance, cognitive performance might
degrade. This study included CVVRT probes with three night time sessions of
about 100 visual stimuli per session delivered randomly to each eye under
computer control. Cognitive degradation may be indicated by an increase in
CVVRT, an increase in missed stimuli or perhaps by the dolphin occasionally
failing to detect stimuli with one eye while continuing to detect with the
other eye. Failure to detect with one eye might indicate uni-hemispheric sleep
or drowsiness.

In the current study, we tested the same female dolphin (SAY) from our
previous study with a male dolphin (NAY) not in our previous study to
determine the effect of continuous vigilance on the dolphin's cognitive
ability. We added a night time CVVRT to determine if: (1) continuous auditory
vigilance had an impact on the speed and accuracy of CVVRT; (2) the CVVRT had
an impact on continuous AGTRT accuracy or speed; or (3) the dolphins, among
all placental mammals having a small corpus callosum relative to brain size
(Tarpley and Ridgway, 1994)
and completely crossed optic chiasm
(McCormick, 1969;
Tarpley et al., 1994), could
transfer learned behavior between their laterally positioned eyes.

MATERIALS AND METHODS

Auditory goal tone response time (AGTRT)

All experiments were conducted in accordance with a protocol approved by
the Institutional Animal Care and Use Committee of the Navy Marine Mammal
Program, Space and Naval Warfare Systems Center, Pacific, San Diego, CA,
USA.

As in our previous study (Ridgway et
al., 2006), we tested two adult bottlenose dolphins (Tursiops
truncatus Montagu 1821). Dolphin NAY (male, age 22, mass 256 kg, length
302 cm) was not in our previous study. Dolphin SAY (female, age 26, mass 246
kg, length 274 cm) did participate in the previous study. The current study
includes seven AGTRT sessions with concurrent CVVRT tasks including four 120 h
sessions (3 SAY, 1 NAY) and three 72 h sessions (2 SAY, 1 NAY). The 120 h
sessions occurred in May (S05), December (S12), March (S03) and November (N11)
while the 72 h sessions occurred in May (S0504), October (S10) and February
(N02). All sessions occurred within an eight month period.

Both animals were exposed to a 0.5 s tone presented every 30 s. The
dolphins' task was to detect a 1.5 s tone of equal amplitude and frequency,
which was infrequently substituted for one of the 0.5 s tones. If the 1.5 s
tone was detected, the dolphin pressed a paddle for food reinforcement. All
tones were the same intensity and projected every 30 s as the dolphins swam
freely in their 9×9 m open air, open water enclosures under ambient
light conditions in San Diego Bay (Fig.
1A).

The dependent variables for vigilance were the time to respond to the
signal and the number of missed or ignored goal tones. Under computer control,
the 1.5 s goal tone stimulus randomly replaced the 0.5 s tone at intervals
between 4 and 24 min. The dolphin was required to press a paddle on one side
of its enclosure within 20.5 s of a goal tone. When the paddle was pressed
within 20.5 s of the goal tone, the computer sounded a buzzer both as a
bridging stimulus (secondary reinforcer) to the dolphin and to signal the
trainer to come out of the equipment hut
(Fig. 1A) and give the dolphin
a fish reward. The rewards were dispensed on a side of the enclosure away from
the goal tone response paddle. If the dolphin pressed the paddle in response
to a 0.5 s tone – a false alarm – the wait period for the next
tone was extended for 30 s.

Behavioral thresholds measured at 1 m from the source
(Ridgway and Carder, 1997;
Schlundt et al., 2000) showed
that dolphin SAY could hear the 70 kHz tones well. She reliably responded to
tones at a level about 20 dB above her audiometric threshold in the bay.
Behavioral tests showed that the male dolphin NAY had a high-frequency hearing
loss. His goal tones were presented at 15 kHz at about 20 dB above his hearing
threshold at that frequency.

The dolphin's approach to the underwater paddle was illuminated at about
850 nm wavelength by infrared (IR) light sources mounted to illuminate both
dolphin eyes and video recorded by IR cameras
(Fig. 1B) on each side of the
dolphin's head throughout all AGTRT sessions
(Ridgway et al., 2006). Much
of the time, the state of both dolphin eyes (open or closed) could be observed
on the recorded video. As in the previous study, the dolphin's behavior was
recorded for a 24 h period before and after one AGTRT session for each dolphin
(S03, N11). Behavior on video during the 24 h periods before was compared with
the 24 h periods after the 120 h session and was analyzed for each dolphin as
an indicator of sleep rebound. The video record was scored for behavioral
indices of sleep (Flanigan,
1974; Goley, 1999;
Lima et al., 2005;
McCormick, 1969;
McCormick, 2007) at 30 s
intervals. Two types of behavior were scored as sleep: slow stereotyped
circular swimming (Flanigan,
1974; Goley,
1999), and floating at the surface with only an occasional beat of
the tail to bring the blowhole above the surface to breathe
(McCormick, 1969;
McCormick, 2007;
Flanigan, 1974).

Experimental setup for the dolphin vigilance sessions. (A) Equipment hut
and test enclosure showing the location of the response paddle and the sound
projector on the adjacent side of the pen where the dolphin was rewarded. (B)
Close-up drawing showing the response paddle apparatus, the underwater
infrared cameras at equal distance on each side and infrared light sources
that allowed for visualization of the dolphin's eyes on goal paddle approaches
for the auditory goal tone response time (AGTRT). The choice
visual–vocal response time (CVVRT) station was put in only at night for
the visual–vocal response time task. (C) Trainer signals the dolphin to
go down to position for CVVRT testing.

Prior to the vigilance sessions, the dolphins were fed a standard amount of
fish each day during the daylight hours – 15 kg for NAY, 12 kg for SAY.
During the vigilance sessions the total daily food consumption was maintained
and spread out over a 24 h period. The animals were fed around-the-clock with
small amounts being given for each correct goal tone response. The trainers
randomized the amount of reward, giving 1–4 small fish for each correct
response so that the day's standard ration (in kilocalories) was delivered by
the end of each 24 h period of the vigilance session. No food was given to the
animals outside of experimental task rewards.

With the female dolphin SAY, we replicated the measures of blood indices
for comparison with the previous results
(Ridgway et al., 2006) in one
of the 120 h AGTRT session (S05). Using voluntary fluke presentation,
non-fasting blood samples were collected for complete blood cell count (CBC),
serum chemistry, catecholamines and selected hormone analyses
(Table 1). Blood was collected
at the same time of the day both immediately prior to the start and at the end
of the 120 h AGTRT session. Samples were analyzed by two laboratories with
experience in performing these specific analyses on dolphin blood (Quest
Diagnostics, San Diego, CA, USA; and ARUP Laboratories, Salt Lake City, UT,
USA). Handling of blood was in accordance with previously established
protocols by Romano et al. (Romano et al.,
2004).

Sixteen blood parameters from blood samples collected before and after
three 120 h vigilance sessions for dolphin SAY

Data were analyzed with Systat 10 (Systat Software Inc., Point Richmond,
CA, USA). Repeated-measures analysis of variance (ANOVA) was used to evaluate
the response time to the goal tone between 24 h periods within the 72 h or 120
h vigilance sessions and between daytime (09:00–16:00 h) and night time
(21:00–04:00 h). Goal tone response time was used as the dependent
variable, and 24 h periods of the vigilance session (1–24 h; 25–48
h; 49–72 h; 73–96 h; 97–120 h), time of day (daytime
vs night time) and animal as the independent variables. Logistic
regressions were used to evaluate changes in the accuracy (correct vs
missed goal tones) by 24 h periods and by time of day (daytime vs
night time) within each session. Probabilities less than 0.05 were considered
significant.

Choice visual–vocal response time (CVVRT)

Both animals learned a choice auditory–vocal response time task for a
previous study (Ridgway et al.,
1991). For the CVVRT task we selected two different stimuli that
appeared in different colors to our human eye – blue/green and red. We
designated blue/green as S1g and red as S2r. Work on color vision in
bottlenose dolphins has been reviewed by Griebel and Schmidt
(Griebel and Schmidt, 2002).
This was not a test of dolphin color vision. The colors were useful for the
trainers and investigators to quickly discriminate the lights and identify
equipment faults that might occur.

Schematic showing the choice visual–vocal response time (CVVRT)
station with positions of the different light stimuli to which the dolphin
responded with a whistle or burst of pulses. To each side are shown the masks
for the red (S1r, S2r) and green (S1g, S2g) that allowed for a different
appearance of each stimulus when the light came on (L, left; R, right).

Dot matrices of blue/green or red LEDs (Lite-on, Milpitas, CA, USA) were
set up as stimuli for the dolphins to discriminate. A black opaque mask 25 mm
in diameter was placed over each LED matrix so that the presentation of
blue/green (S1g) was a 10×20 mm blue/green bar through the center of the
mask (Fig. 2). Stimulus 2 (S2r)
was a 50×70 mm LED matrix that appeared red to the human eye. S2r had
three equally spaced horizontal bars of red light. The center bar was
4×20 mm with side bars of 4×13 mm above and below the center bar
(Fig. 2).

Bottlenose dolphin light spectral sensitivity has previously been measured
in the range of 397–636 nm with a sensitivity peak of 490 nm
(Griebel and Schmidt, 2002). A
recent review (Peichl et al.,
2001) noted that T. truncatus and many other marine
mammals have L-cones but not S-cones. At least two types of cones are required
for color vision and dolphins were found to be `essentially color blind'.
Therefore, we were determined not to attempt a test of color vision but to
design stimuli for our CVVRT task that the dolphin could readily discriminate
based on any one of several characteristics.

With the light stimuli configured as shown in
Fig. 2, a monochromator was
used to measure spectra and luminous flux at a distance of 25 cm from each
stimulus. This was the distance of each stimulus from each dolphin eye. S1g in
the blue/green region had a peak of about 480 nm with a range of roughly
440–550 nm (Fig. 3A)
whereas S2r had spectral peak of 630 nm and a range of roughly 580–670
nm (Fig. 3B). The luminous flux
of S1g was 0.5 mlu nm–1 and S2r was 2.5 mlu
nm–1. In this configuration, the dolphin had clues other than
color for discrimination. These included the different stripes through the
mask aperture, the difference in brightness, the slightly different positions
of the two stimuli (Fig. 2) and
possibly the different spectral characteristics
(Fig. 3A,B). Each dolphin
retina has two areas of ganglion cell density
(Mass and Supin, 1995). This
feature may aid in differentiating stimulus position.

Dolphin SAY was first trained to respond to S1g with the right eye while
the left eye was covered with a rubber suction cup. Training the vocal
responses has been previously described
(Ridgway and Carder, 1988;
Ridgway et al., 1991;
Ridgway et al., 2001;
Schlundt et al., 2000).
Briefly, S1g presentation to the right eye was paired with a tap on the left
margin of the nasal plug eliciting a whistle from the animal. The S1g
presentation was put under computer control with custom software. The program
output a stimulus with a uniform duration of 109 ms so that S1g onset time and
hydrophone reception of the whistle onset could be recorded and response time
(CVVRT) calculated and stored (Fig.
4A,B). After SAY performance exceeded 90% correct responses, the
suction cup was removed from the left eye and placed on the right so that S1g
could now be presented only to the left eye. Yaman et al. have presented data
and reviewed literature suggesting right eye dominance in bottlenose dolphins
(Yaman et al., 2003).

(A) Plots of monochromator readings of luminous flux in millilumens per nm
(mlu nm–1) vs wavelength (nm) for three of the units
used as S1g stimuli and (B) S2r stimuli. Note that the peak of luminous flux
for S1g was less than one–fifth of that of S2r.

Examples of the 2 s window provided for dolphin SAY to respond to a S2r
stimulus with a correct burst pulse vocalization above and an S1g stimulus
with a correct whistle response below. NAY responded to S2r with a whistle and
to S1g with a burst pulse sound.

The vocal response to S2r was a burst pulse sound first elicited with a tap
on the right side of the blowhole. The burst pulse was paired with S2r. The
same procedure was used for the 109 ms S2r presentation first training the
right eye with the left eye covered and then training the left eye with the
right eye covered. Dolphin NAY was trained in the same manner except that his
responses were reversed from those of dolphin SAY; S1g elicited a burst pulse
and S2r a whistle.

We conducted CVVRT trials only during dark hours between approximately
20:00–03:00 h. Each CVVRT session lasted about 20 min and consisted of
several brief sittings during which the dolphin stationed
(Fig. 1C;
Fig. 2) to receive a series of
light stimuli. Three of these 20 min sessions were conducted each night. A
sitting began with the trainer pausing the AGTRT program, then signaling the
dolphin down to a station 1 m underwater
(Fig. 1C). When the dolphin was
positioned correctly (Fig. 2),
the trainer pushed a switch beginning the CVVRT computer program that
presented the S1g and S2r stimuli to each eye with an inter-stimulus interval
of 4.1–8.2 s randomized in 0.1 s increments. The dolphin had to remain
on station for about one minute and attend to four different light sources,
two on the right and two on the left (Fig.
2). The computer program established a 2 s window following the
onset of each 109 ms light stimulus to record dolphin vocal responses from the
hydrophone (Fig. 4A,B). The
computer program controlled stimulus presentation in a random order and the
trainer was not aware of stimulus order. After a series of several responses,
an operator in the computer hut switched on an underwater buzzer immediately
following a correct response in the series near the one minute stationing
period. The buzzer was a signal to the dolphin to surface and a signal to the
trainer to reward the animal with fish. No responses and responses outside the
2 s window were counted as misses. Response time for correct trials to the
visual stimulus within each AGTRT session was used as the dependent variable
with the night and stimulus (S1g, S2g, S1r, S2r) or stimulus side (left
vs right) as the independent variable in repeated-measures ANOVA.
There was no attempt to train the animal to respond as fast as possible to the
light stimuli.

The impact of the night CVVRT trials on the dolphin's ability to maintain
continuous auditory vigilance was also assessed by comparing the AGTRT
accuracy and response time with the goal tone during the four hours prior to
and the four hours following the CVVRT trials. Logistic regressions were used
to evaluate changes in accuracy by time of trial (before vs after the
visual task) and night. For these analyses, accuracy (correct vs
missed) was used as the dependent variable with night and time of trial as the
independent variables.

RESULTS

The training procedure with one eye covered with an opaque suction cup
allowed us to determine if the response learned with right eye stimulus
correct response transferred to the left eye and vice versa. For both
stimuli (S1g and S2r), transfer was almost immediate requiring only one to
three trials.

Results of blood measures taken before and after one 120 h AGTRT session
for SAY are presented in Table
1 and did not differ from previously reported blood values during
AGTRT (Ridgway et al.,
2006).

Effect of night time CVVRT tasks on AGTRT

Mean AGTRT for each 24 h period of the three 72 h and four 120 h AGTRT
sessions are presented in Table
2. Mean response time increased slightly for each of the first
three 24 h periods and then declined slightly for the last two 24 h periods of
each the 120 h session (Fig.
5A). A similar trend was found in the 72 h AGTRT sessions, with a
slight increase in the mean response time during each of the three 24 h
periods (Fig. 5B). However, the
slight changes observed over the course of the AGTRT sessions were not
significant for the 120 h (F=3.503, P=0.062) or 72 h
(F=0.528, P=0.655) sessions. Numerical data and percentage
of correct responses for each 24 h period for all seven AGTRT sessions are
shown in Table 2. There was no
change in accuracy over the course of any of the 72 h or 120 h
(logit<1.389, P>0.185) AGTRT sessions when compared by 24 h
period.

Descriptive statistics for each 24 h period of each 72 h and 120 h AGTRT
trial

The significant diurnal pattern of AGTRT observed in the previous 120 h
continuous auditory vigilance sessions
(Ridgway et al., 2006) was not
as obvious in the AGTRT of the two dolphins in the present study. There was no
significant difference between goal response time between the night time
(21:00–04:00 h) compared with daytime (09:00–16:00 h) for the 120
h (F=2.800, P=0.236) or 72 h (F=5.044,
P=0.267) sessions. Nor was there a difference in accuracy between the
night time (21:00–04:00 h) compared with daytime (09:00–16:00 h)
during any of the 72 h or 120 h (logit<1.288, P>0.280) AGTRT
sessions.

The AGTRT for correct responses did not significantly change during the
four hours following the CVVRT trials compared with the four hours before the
CVVRT during the 120 h session (F=2.627, P=0.352) nor did it
change over the course of the AGTRT session (F=1.480,
P=0.357; nights 1–5). Further, there was no change in accuracy
of the AGTRT session during the four hours following CVVRT (logit>3.979,
P>0.313).

In the 72 h sessions, AGTRT during the four hours following the CVVRT
trials did not significantly change compared with the four hours prior
(F=2.978, P=0.334) nor was there an increase in missed
trials (logit>0.00, P>0.348). However, the AGTRT during the
four hours after the CVVRT trial did significantly slow over the course of the
session (F=27.619, P=0.035; nights 1–3).

When the 24 h before the start of the AGTRT session was compared with the
24 h following the AGTRT sessions (Fig.
6) there was not a substantial increase in sleep behaviors
(Flanigan, 1974;
Goley, 1999;
McCormick, 1969) or signs of
sleep deprivation (Dinges et al.,
1994; Dukas and Clark,
1995; Oleksenko et al.,
1992); however, dolphin SAY did show a slight increase in `sleep'
time after one of the 120 h vigilance sessions (S03). In contrast to NAY's
night time behavior, SAY floated less and usually continued to circle the
enclosure. Especially at night this circling became very slow and stereotyped
as has been previously described by Flanigan
(Flanigan, 1974) and Goley
(Goley, 1999).

Eye closure could not always be determined from our IR illuminators and
underwater camera system. Video records from cameras on each side of the head
with views of both eyes never clearly indicated simultaneous closure of both
eyes (Fig. 7). The dolphin
might approach the goal paddle at any angle, thus recognition of eye state
(open or closed) was especially difficult at night.

Effect of continuous AGTRT on the cognitive (CVVRT) task

Of the 29 different 24 h periods during which CVVRT trials were conducted,
there did not appear to be a difference in accuracy of response between the
two dolphins. Mean accuracy (% correct responses) are shown for each 24 h
period in Table 3. Overall,
mean CVVRT accuracy for all 24 h periods for SAY were: left S1g, 95.6%; left
S2r, 94.1%; right S1g, 87.8%; and right S2r, 95.9%. For NAY, CVVRT accuracy
means were: left S1g, 92.4%; left S2r, 90.3%; right S1g, 92.2%; and right S2r,
89.2%.

Descriptive statistic for onset of vocal response (s) by stimulus for each
24 hour period

There was a difference in CVVRT between the two animals, and between the
different AGTRT sessions with the same animal
(Table 3). However, the CVVRT
did not significantly slow or degrade over the course of the 72 h
(F=4.188, P=0.104; nights 1–3) or the 120 h
(F=2.298, P=0.119; nights 1–5) AGTRT sessions. Nor was
there a difference in CVVRT between stimulus sides (left vs right
eye) over the course of the 72 h (F=2.319, P=0,267) or 120 h
(F=4.566, P=0.122) AGTRT sessions. Accuracy of the CVVRT did
not significantly change over the course of any of the AGTRT sessions
(logit>1.131, P>0.191).

DISCUSSION

This was the first test of inter-ocular transfer of learned behavior in
dolphins or any cetacean. The position of the dolphin eyes on the side of the
head suggested the need for a test of inter-ocular transfer. Dolphins only
have a small binocular field of view
(McCormick, 1969;
McCormick, 2007). Furthermore,
with similar lateral eye positions, rabbits and pigeons sometimes fail to
transfer information learned in one eye to the other
(de Vos-Korthals and Van Hof,
1983; Graves and Goodale,
1977; Jimanez Ortega et al.,
2008). Although dolphin pupils respond consensually to bright
light presented unilaterally (Lyamin et
al., 2008), dolphin eyes may move independently, one orienting
backward while the other orients forward
(McCormick, 1969), and
measured eye movements are slow (Dawson et
al., 1981). Author Herman Melville spent many hours observing and
thinking about cetacean behavior aboard the whaling vessel Acushnet.
In Moby Dick he wrote about the sperm whale's laterally spaced eyes.
Melville noted:

The whale, therefore, must see one distinct picture on this side, and
another distinct picture on that side; while all between must be profound
darkness and nothingness to him. Nevertheless, any one's experience will teach
him, that though he can take in an indiscriminating sweep of things at one
glance, it is quite impossible for him, attentively, and completely, to
examine any two things – however large or however small – at one
and the same instant of time; never mind if they lie side by side and touch
each other. But if you now come to separate these two objects, and surround
each by a circle of profound darkness; then, in order to see one of them, in
such a manner as to bring your mind to bear on it, the other will be utterly
excluded from your contemporary consciousness. How is it, then, with the
whale?... is his brain so much more comprehensive, combining, and subtle than
man's, that he can at the same moment of time attentively examine two distinct
prospects, one on one side of him, and the other in an exactly opposite
direction.Chapter 74, p. 329 in Moby-Dick
(Melville, 1851)

Behavior recorded in the 24 h before and the 24 h after two of the 120 h
continuous vigilance auditory goal tone response time (AGTRT) trials for SAY
(A) and NAY (B). Slow circular swimming (SCS) and quiescent hanging behavior
(QHB) are associated with sleep or resting.

Although the dolphin corpus callosum is small relative to brain size, the
posterior commissure is large relative to that seen in most mammals
(Lyamin et al., 2008). Also
the inter-collicular commissures are present. This anatomy could at least
partially explain why the inter-ocular transfer of the unilaterally learned
behavior of the visual–vocal response was almost immediate despite the
complete decussation of the optic nerve at the chiasm and the lateral position
of the eyes.

Since the dolphin optic nerve is completely crossed at the optic chiasm
(McCormick, 1969;
Tarpley et al., 1994) visual
input to the left eye is transmitted directly to the right brain hemisphere
and vice versa. In addition, the dolphin has, among placental
mammals, a relatively small corpus callosum relative to brain size connecting
the two brain hemispheres (Keogh and
Ridgway, 2008; Tarpley and
Ridgway, 1994). These anatomical arrangements influenced our
choice of a CVVRT task presenting two different stimuli in a random order to
the dolphins' left and right eyes. Dolphins in uni-hemispheric sleep have been
shown to have uni-hemispheric slow waves from electrodes over the cerebral
cortex, including visual cortex, as well as in the thalamus
(Mukhametov, 1984;
Lyamin et al., 2008). If a
dolphin had been in uni-hemispheric sleep mode during these visual trials of
the CVVRT task, there should have been a marked asymmetry in the speed or
accuracy of the CVVRT task between the two eyes. Our results did not show any
such asymmetry during the CVVRT task (Table
3).

The dolphins showed no indications of a vigilance decrement during the 72 h
or 120 h continuous vigilance sessions. This was shown by the: (1) accuracy of
AGTRT during each session; (2) AGTRT latency over the course of trial; (3)
correct response accuracy to the CVVRT trials each night; and (4) maintenance
of response speed in the CVVRT cognitive task. In human vigilance trials,
degradation in both physical and cognitive performance have been consistently
observed. Human cognitive ability, as measured by choice response time,
degrades more rapidly than physical performance during periods of continuous
vigilance (Dinges et al., 1997;
Liberman et al., 2006). A
reduced amount of sleep resulted in a degradation of human response time
(Lim and Dinges, 2008) and
performance on a response time task. The dolphins' lack of a significant
decrement in auditory vigilance or in a cognitive task over 72 h or 120 h
sharply contrasted with both field observations and laboratory studies of
other species (Dinges et al.,
1997; Dukas and Clark,
1995; Liberman et al.,
2006; Siegel,
2005). It is an almost universal finding that prolonged periods of
continuous vigilance or sleep deprivation of even shorter periods than our
dolphin trials cause a reduction in vigilance performance
(Beaumont et al., 2001;
Davies and Parasuraman, 1982;
Gilberg and Akerstedt, 1998; Horne and
Pettitt, 1985). Cognitive performance degrades even more rapidly
(Dinges et al., 1997;
Liberman et al., 2006).

Eye closures observed on video as dolphin SAY approached the target
response paddle. All closures (save 1) involved only one eye and all were
during daytime hours. Gray areas denote time between sunset and sunrise.

Sleep deprivation changes result from many human clinical blood
measurements (Dinges et al.,
1994; Heiser et al.,
2000; Suchecki et al.,
1998). Using the same measurements as had been used for humans, we
quantified 57 parameters from whole blood, serum and plasma samples taken at
the start and at the end of one 120 h continuous vigilance session for dolphin
SAY. For example, leukocyte measurements, cortisol, epinephrine,
norepinephrine and dopamine showed no drastic changes before and after the
vigilance session and all parameters of interest remained within normal ranges
(St Aubin et al., 1996;
Venn-Watson et al., 2007).
These results (Table 1) are
consistent with the previously reported 120 h continuous vigilance sessions
(Ridgway et al., 2006).

Sleep rebound is a universal finding in sleep deprivation in humans and
other mammals (Benington and Heller,
1999; Tobler,
1985). Neither dolphin showed marked sleep rebound after any of
the 120 h vigilance sessions as indicated by our observations of behavior and
posture (Flanigan, 1974;
McCormick, 1969;
McCormick, 2007) comparing the
24 h before and after the 120 h vigilance session
(Fig. 6). Although dolphin SAY
did demonstrate more `sleep behaviors' after the 120 h vigilance session for
which these behaviors were quantified, the amount of such post-vigilance
behavior was within the range of that was observed previously
(Ridgway et al., 2006) for the
same animal before and after 120 h vigilance sessions.

Dolphin light sensitivity and complete crossing of the optic
nerve

Some birds have adopted uni-hemispheric sleep to maintain visual vigilance
(Lima et al., 2005;
Rattenborg et al., 1999). The
dolphin's ability to sleep with one hemisphere and to have each brain
hemisphere sleeping alternately (Lyamin et
al., 2008) seems the most likely explanation for the dolphin's
ability to detect and respond to the acoustic goals randomly presented over
the continuous three or five days of these sessions without signs of sleep
deprivation.

Goley (Goley, 1999)
observed night time eye closures by visible light; however, an animal with
both eyes below the surface at night where visible light is minimal may have
little need for eye closure to reduce illumination that might interfere with
sleep. Based on multi-lead electroencephalographic (EEG) recordings from both
hemispheres, Mukhametov (Mukhametov,
1984) suggested that the open dolphin eye could perform a sentinel
function `regardless of whether the contralateral hemisphere was asleep or
awake'. When `checkerboard' square stimuli were presented to an open eye of
sleeping dolphins there was a rapid startle response or other behavioral
reaction (Lyamin et al.,
2008). However, during uni-hemispheric sleep the eye contralateral
to the hemisphere exhibiting slow waves was usually closed
(Lyamin et al., 2008).

When the dolphins in our study were slowly swimming around their enclosure
at night we could not determine their eye state; however, video did record eye
state during approaches to the goal paddle in response to the goal tones.
During EEG recordings of dolphin uni-hemispheric sleep, as indicated by slow
waves, the eye contralateral to the sleeping hemisphere was closed about 75%
of the time in the studies of Lyamin et al.
(Lyamin et al., 2004). In the
current vigilance studies, we replicated the findings of our previous work
(Ridgway et al., 2006).
Closure of either eye was observed only during daytime goal paddle approaches
(Fig. 7). Bright sunlight
penetrating surface waters during daylight hours or visible light used for
observation at night might influence the need for eye closure. We used IR
illumination at 850 nm to illuminate the dolphin when at the surface and
underwater approaching the goal paddles because these events were not visible
to our trainers without the IR system.

The absence of eye closure during night time goal approaches observed
during this study (Fig. 7)
suggests that if the dolphin were asleep in either hemisphere with one or both
eyes closed, the AGTRT tone aroused the animal. Whether the dolphin was asleep
in both brain hemispheres (McCormick,
1969; McCormick,
2007; Ridgway,
2002; Lyamin et al.,
2008) or in only one brain hemisphere
(Mukhametov, 1984;
Ridgway, 2002;
Lyamin et al., 2008) it was
probably aroused by the goal tone. This arousal lasted at least for the
approach and paddle press followed by collection of the fish reward near the
hydrophone station on the adjacent side of the enclosure
(Fig. 1A,B). During the night
time periods, the dolphin's posture and behavior was often consistent with
rest (Flanigan, 1974;
Goley, 1999;
Lima et al., 2005;
McCormick, 1969) except during
the brief periods of 25 s or so required for goal approach and taking of the
food reward. Slowed AGTRT response times
(Fig. 6A,B) could be viewed as
an increased arousal threshold. If the slowed response time was an indication
of increased arousal threshold, it suggests that the goal tone response
involved an arousal even from uni-hemispheric sleep. The dolphin might have
been able to process the auditory cue, awaken and initiate the behavioral
response. The dolphin auditory system is well connected to brainstem alerting
systems. Still, it seems remarkable that the dolphin was able to discriminate
the goal tone from the non-salient background tones, swim to the goal tone
response paddle, then swim to the adjacent side of the enclosure and eat the
fish reward and repeat this 25 or 35 times during the night time period and
show no signs of sleep deprivation or cognitive impairment.

It is apparent that dolphins were able to maintain cognitive ability as
well as to detect goal tone stimuli at intervals during long vigilance trials
to at least 120 continuous hours. The ability of the dolphin to obtain
sufficient sleep by uni-hemispheric means during intervals when a behavioral
or vocal response is not required, most probably explains the dolphin's
vigilance capability.

From a cognitive standpoint, dolphins rapidly detected a visual stimulus
and give a separate and distinct vocal response to two different lateral
visual stimuli. The dolphins attended and responded correctly even though the
stimuli alternated randomly between the laterally positioned eyes. How many
distinct vocal responses can be made by a dolphin to a larger number of
different visual stimuli is still to be determined.

FOOTNOTES

The animal care and training staff of the Space and Naval Warfare Systems
Center were critical for the training and care of the dolphin subjects. We
thank Pete Poirier and Mark Lasher for our LED photometric measurements. Drs
Mark Xitco, Eric Jensen, Cynthia Smith, John Carney, Brett Giroir, Amy Kruse
and Lisa Ely for support and encouragement. This study was supported by the
Defense Advance Research Projects Agency
(DARPA) under SSC San Diego Contract N66001-05-C-0040.

Ridgway, S. H. (2002). Asymmetry and symmetry
in brain waves from dolphin left and right hemispheres: some observations
after anesthesia, during quiescent hanging behavior, and during visual
obstruction. Brain. Behav. Evol.60,265
-274.

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